A massive mystery

During the past century, physicists have built an overarching theory called the Standard Model to explain a zoo of subatomic particles they've found and the roles these particles play in nature. One final particle the theory predicts, however, remains missing in action.

This month, researchers at the European Laboratory for High Physics (CERN) are driving an enormous particle accelerator to its limits and beyond in an effort to search for the particle: the Higgs boson. Finding it would confirm a 30-year-old theory about why matter's most fundamental particles have mass.

"The discovery of the Higgs boson would mark a profound point in the history of science," notes Sau Lan Wu, a physics professor at the University of Wisconsin at Madison and a collaborator on one of four detectors CERN is using in the hunt. "There is literally no other particle like it, and without it our understanding of the behavior of matter and energy at the most fundamental levels breaks down.

Discovering the Higgs boson, if it exists, would help answer a question that has vexed physicists for years: What gives nature's fundamental particles mass? Without mass, these particles would flit about the cosmos, sprinting past each other at the speed of light. Interactions, if they took place at all, would be extremely rare. Absence of interactions means no matter as we know it - no galaxies, stars, or planets. This postulated role for the Higgs boson has prompted physicist Leon Lederman, himself a Nobel Prize winner, to dub it "the God particle."

The researchers at CERN are not alone in their hunt. The Fermi National Accelerator Laboratory (FNAL) in Batavia, Ill., is putting the finishing touches on $420 million in upgrades to its accelerator and two detectors in preparation for a range of high-energy physics experiments, including the hunt for the Higgs.

Once CERN's run ends Nov. 2, Fermilab will be the only facility capable of searching for the particle; the collider CERN is using is scheduled to shut down so its tunnel can be used for a new, more powerful accelerator the lab hopes to complete by 2005. If Fermilab's accelerator, currently the world's most powerful, fails to nab the Higgs, CERN's new $6 billion machine will, physicists say.

The four forces of nature

The Higgs boson was first proposed by Scottish physicist Peter Higgs in the mid-1960s to help solve a puzzle that emerged as physicists began to see how the four forces of nature - (1) electromagnetism, (2) the strong force (which binds atomic nuclei), (3) the weak force (which governs radioactive decay and some fusion reactions), and (4) gravity - might be low-energy manifestations of one force that briefly held sway during the universe's earliest moments.

A key milestone along the trail toward "grand unification" came in 1961, when physicists Sheldon Glashow, Steven Weinberg, and Abdus Salam developed a theory that successfully married electromagnetism with the weak force. These forces have particles associated with them: photons to "carry" electromagnetism and W and Z particles to carry the weak force. At first glance, it might seem as though particles with similar roles should share critical attributes. But the weak force acts over very small distances, while electromagnetic fields permeate the universe. This meant that the particles associated with the weak force must have substantial mass. Researchers at CERN discovered the W and Z particles in 1983. Nobel Prizes went to the three theorists, as well as to Carlo Rubia and Somin van der Meer of CERN for finding the particles.

Yet the newly minted "electroweak" theory left open the question of why particles that should exhibit "symmetrical" properties displayed such different masses. Photons are essentially massless, a W or Z particle tips the scales at about 100 times a proton's mass.

In the mid-1960s, Dr. Higgs proposed an answer: The universe was permeated with a field that imparts mass to particles. A particle's mass depended on how strongly it interacted with this field. Under this idea, photons rarely, if ever, interact with the Higgs field, while the W and Z particles interact very strongly with the field. Such a mass-inducing field would have a "force-carrying" particle associated with it - the Higgs boson.

The Higgs boson itself is no lightweight, notes Meenakshi Narain, a physicist at Boston University who collaborates on one of the two detectors Fermilab will use for the Higgs search. In the simplest version of the Standard Model, the Higgs boson mass should appear at somewhere between 90 and 160 times the mass of a proton, making it one of the heaviest particle physicists have yet sought. "That's well within Fermilab's range," she says.

Such massive particles require accelerators that can generate collision energies at least as high as those represented by the particle's mass.

Fermilab's accelerator, called the Tevatron, has two detectors. Weighing in at 5,000 tons each, the detectors record the energy, momentum, and lifetime of the debris that bursts from collisions between protons and their antimatter equivalents, antiprotons. The addition of a new system to inject the proton beams into the Tevatron means that the experimenters will have 10 times more collisions than before the modifications, or about 1 million collisions each second.

Yet if hunting the Higgs requires energy, it also requires patience. Rutgers University physicist John Conway notes that, out of 1 million collisions each second, state-of-the-art data-processing techniques allow scientists to capture only about 500 collisions a second.

After two years, he adds, researchers will have "perhaps a few tens of events" to use either to confirm that CERN has seen evidence of the Higgs boson or establish that CERN's results were a statistical fluke. If the Higgs boson does lie in Fermilab's energy range, he adds, researchers should see it by 2005 or 2006.

If the Higgs boson fails to appear in Illinois, a Nobel will likely go to CERN. If it does appear in Illinois, CERN's new machine - the Large Hadron Collider - would open the way for more-detailed studies of the Higgs boson.

Indeed, for all the interest in finding the Higgs boson, not finding it would be "even more interesting," says Elizabeth Simmons, a physicist at Boston University.

She and others note that Higgs boson is a messy answer to the problem of mass. "The Higgs field was an ad hoc solution," says Franco Bedeschi, a physicist at Fermilab. "When you add it, it gives masses to particles. But it's not a natural solution; it doesn't come from first principles."

Finding the consistent story

Physicists can take several approaches to describe the Higgs mathematically, Dr. Simmons says, but they should "tell a consistent story." The trouble emerges when physicists use quantum mechanics to describe the Higgs boson; its mass grows by 10 million billion times. Such an enormous gulf between the Higgs' mass and that of the W particle would defy explanation.

"But its worse than that," she continues. Using quantum approaches to calculate how the Higgs field's influence changed as the universe cooled, under today's condition its ability to impart mass would have eroded to virtually zero.

These problems suggest that the quest for the Higgs may in fact be a quest for "new physics" that extends beyond what the "standard model" can now explain. Alternatives include supersymmetry and string theory, which posit the possibility of several Higgs bosons at different mass ranges.

Another, known as technicolor, suggests that the Higgs boson is not a fundamental particle, but may be a manifestation of deeper processes that carry their own particle like signatures - much as protons and neutrons were found to consist of quarks and were no longer viewed as fundamental particles.

Perhaps the most unsatisfying aspect of the Higgs mechanism is its inability to answer the most fundamental question, adds Dr. Conway. Perhaps it can explain how fundamental particles acquire mass, "but it doesn't tell us why different particles have different masses."